It started as it so often does whenever, after a nice weekend filled with music from our borrowed test boxes, Monday rolls around again. Boris had subjected the SB 18 to a test at home and thought that everything in my sound description was realistic. He was very impressed by the detailed contrast and the harmonic interactions of the SBAcoustics chassis. Now, his listening room is fairly large, and there’s plenty of room for a free-standing box. If he ever ran across the famous good fairy, he said, he would only have one wish to make: just a little more pressure in the bass – then he could say he had found his box. To keep him from waiting for all eternity, I invented the SB 36 for him.
As we know, size is relative, and even in a large room there’s much more to set up than just a couple of overgrown loudspeaker boxes. So Boris refused right away when I suggested putting an SB29NRXC75-6 under the SB 18 on each side, which would give him a -3dB point of less than 30 Hz with just under 90 reflex-tuned liters. Too big, he said. He couldn’t sacrifice more than the footprint of the SB 18, and it didn’t need to be much deeper. He just wanted a little more air movement. That made the structure of the boxes clear, and all I had to do was add a second SB17NRXC35-8 to the SB 18, which already includes the ferrofluid-free SB26STC-C4 doing its work as a tweeter, which is worth at least twice its very reasonable price.
It also gives me the opportunity to finally clear up an old hope that is far too often associated with dual-bass systems. The second chassis doesn’t give you more bass depth if the volume is simply doubled. Nonetheless, in direct comparisons that’s what it feels like. The larger membrane area moves more air, which comes toward the listener across a wider front and thus seems like it has more pressure. Aha – wasn’t that the wish for the good fairy? That’s how the SB 18 became the SB 36. Coincidentally, the suffix also shows the volume of enclosed air in liters.
The complete data sheets, along with the measurement value download, can be found here. The membrane of the BMT looks slightly different now – it comes in a solid black, without the gray specks and thus slightly more neutral – but the parameters for the chassis have remained almost identical, within the scope of the normal tolerances.
There are also a few things worth mentioning about the “dual bass” cabinet type, so we will take some time for that here. First, it is always a matter of debate whether every bass needs its own chamber in order to work with a precisely defined volume, or whether both basses should share a home. In my experience there is no perceptible difference, even if there are minimal physical distinctions. In my case, I decided that the benefits of the shared reflex tube, which would be in the front because it caused fewer setup problems, made it worth having an undivided structure.
The second question that comes up when designing a box with two basses has to do with the chassis placement. The D’Appolito look is popular – it involves putting the tweeter between the two parallel bass mid-range speakers. That creates a kind of single-point sound source, since the sound from all three chassis elements meets at the level of the tweeter. The result is the wonderful spatial reproduction that wide-range speakers are known for. There are two downsides to this structure: above and below the tweeter axis, the sound-pressure curve is very wavy because the paths from the lower and upper mid-range speaker to the listener’s ears have different lengths. That causes breaks in some of the frequencies. As a result, the box needs to be tall enough that the music consumer sitting in front of it is looking right into the tweeter.
The alternative is the almost traditional chassis structure in which the basses are stacked under the tweeter. Because of the different distances that this creates to the ear, the lower driver is only used to supplement the low notes, while the upper driver transmits the mid-range. This setup is commonly known as the two-and-a-half-way box, and the benefit is that the lower range can be emphasized more or less depending on the uncoupling of the support bass and according to personal taste. That was my compromise, which Boris immediately accepted in part also because of the lower height.
SB36 construction plan as a Sketchup-file
My cabinet design once again followed the motto: make it as easy as possible. I had four identical boards for the front, sides and back wall, along with two matching lids and floors, cut to size from black MDF for each box. Because of the small interior surface and the heaviness of the boards, I was even able to leave out the reinforcements with a clear conscience. By contrast, Boris made a little more work for himself; he built an inner cage with rounded sides and then glued an alder veneer to the outside. He also decorated the front and back walls with edges and curves, and stained them black to provide contrast. Unfortunately he didn’t take any pictures of his more ambitious woodwork, so my readers will have to be satisfied with my documentation of the gluing process yet again.
There’s not much more to say about gluing six boards with joint glue, so I’ll just give you the sequence as an overview: I glued the first side, the lid, the floor and the second side to the front wall. The glue is always applied to the cut edge, and the individual boards are flush with each other. Once the glue dries, the boxes are carefully sanded so that no traces of glue are visible when the clear varnish is put on. In order to make the cut edges a little less obvious, I separated them from the adjacent boards with a shadow gap. I liked it so much that I applied the same visual loosening to the front, too. I had bought a triangular trimming cutter for this job many years ago; it was first used for the article on the FirstTime 10 in the February 09 issue. That issue also shows how to cut out the chassis holes, so I will skip any further explanations. I gave the wood three coats of Hornbach acrylic PU paint with my Wagner W 660 sprayer, and I did an intermediate sanding step by hand with a #240 abrasive sponge. Given the low level of effort involved, the results are more than satisfactory.
The crossover network
The frequency response for the two parallel-connected SB 17s doesn’t look too bad if we can just ignore the strong waves above 900 Hz (green). They are the result of the different distances from the microphone (standing in for the ear in the measurements) that I mentioned earlier. A large coil in front of the lower bass, whose effects can be seen even at 100 Hz and up, removes the spikes. It can be used to “set” the voice reproduction; at higher values, men sound slimmer, and at lower values they are fuller. As always, we chose the happy medium as a neutral compromise, and we recommend that do-it-yourselfers try it out for themselves to see what works best in their homes. Don’t worry, you won’t break anything – you can’t break off any connection wires with coils.
With the SB 18 I had more or less followed the recommended connections from SBAcoustics, but I gave my imagination free rein in developing the mid-range section of the SB 36. The upper bass mid-range got a low-pass filter (a filter that lets low frequencies through) made of a Corobar coil with an overlaid MKP Q4 capacitor that reduced the peaks above 3.5 Hz; behind it was a smooth electrolytic capacitor parallel to the chassis. The tweeter was given just as many upstream components as the two big chassis elements; its volume level was reduced by two Mox 4 resistors, and a capacitor and a small air coil protected it against too-deep frequencies of 12 dB/octave below 2.8 kHz. The total frequency response dips a little bit around 3 kHz, thanks to the width of the baffle board, which no longer reflects the high-tone signals that are radiated in all directions.
You can see how well the crossover is tuned by looking at the addition of the amplitudes under 0, 30 and 60 degrees; all of the dips and peaks are nearly balanced out. Naturally, we also smoothed the impedance for people who like to pamper their ears with tube systems.
To clarify the circuitry, we placed the components on the crossover plan; the next image shows them glued onto a wooden board and welded together. Those who prefer a slightly cleaner look can also assemble them on a universal grid-style board. The three impedance correction components are ideally placed either inside or outside, at the terminal to which they are connected in parallel.
With five bags of Sonofil, the insulation volume is pretty significant. The damping plan, for which there is not enough space here, shows you how to insert the insulation.
Who could really fault me if I just used the exact same text as my sound description for the SB 18, unabridged and just with the phrase “even more pressure!” despite the completely new crossover topology? I could hardly provide a more accurate review of this assembly kit, which simply includes an additional bass. And yet the SB 36 does offer a slightly different quality; it really does sound much more grown up now, with more pressure in the bottom range and more apparent depth. Even if the SB 18 already reproduces music very convincingly, it naturally has its limits when compared to its big sister. The kettledrum was a bit bigger and of course had more pressure, but the SB 36 didn’t lose any of the fine details that had earned the little single-bass box the “Blues class” label, very rare for its price range. The SB 36 didn’t show any sign of pudginess in the bass or the lower mid-range, which all too often ruins the fun of the music. The fact that singers gain stature over the course of many years became clear in listening to the slim free-standing box, when Leonard Cohen performed his famous “Suzanne” first as a studio recording and then, immediately but forty years later, on “Live in London 2008,” as if he were a time traveler making two visits to Kerpen near Cologne, Germany. Once a young man with an already fairly sonorous voice, he now sounded like he had just been waiting for the opportunity – now at age 75 and with his voice half an octave lower – to tell the story of the young woman from the port of Montreal one more time, with more expression. The tweeter was outstandingly matched with the lower section. Calmly and with great self-assurance, lacking any sense of pushiness, it played its role in the outstanding performance. For at least two and a half hours of pure emotion, the old man and great artist transformed the listening room with his poetic texts and fairly simple melodies – and I enjoyed every minute of it.
We’re not trying to hide Boris’ SB 36 version – it deserves to be seen. Instead of a tube, he preferred a slit under the front as a reflex outlet; the baffle board was covered in black synthetic leather.
|Chassis||2 x SB17NRXC35-8||Wood list in 19 mm MDF per piece:|
|95,0 x 22,0 (4x) front/ back wall/sides|
|Sales||Intertechnik||18,2 x 22,0 (2x) lid/floor|
|Principle||Bass-reflex||Bass.Midrange: 6 mm|
|Nominal impedance||4 Ohm||Tweeter: 3mm|
|Damping:||5 bags Sonofil|
|Pole clamp/Terminal||K 30-AU||Wood cutting black MDF: 40 EUR 42 USD|
|Reflex port:||BR70/HP unshortened|
|Approx. cost per Box:|
|Kit without wood||230 EUR 245 USD|
Expandability of the SB 36 kit:
- SB36 can be expand with 1 x SB36center (as center speaker)
- SB36 can be expand with 2 x SB18 (as rear speaker) or
- SB36 can be expand with 2 x SB36 (as rear speaker)